You want a headset that keeps up with games for a few years so you aren't buying another one next winter. Wait for the holiday sales when retailers dump old stock to make room for new gear.
Last year's model is usually your best bet. It plays the same games as the new version for way less money. Don't worry about missing out on minor hardware tweaks.
That 18-month window is a deadline, not free money. If you have any balance left after the 18 months are up, you will start paying interest on whatever is still sitting on the card.